Chapter 7 argues that a spouse should not be loved as private property but as a representative of all men or all women. Marriage does not shrink love down to one person; it teaches a person how to love the wider world through one chosen representative.
That gives conjugal love a universalizing function. The husband or wife becomes a concentrated form of humanity to whom one learns fidelity, service, delight, and reverence.
This is one reason marriage matters for the larger Kingdom vision. A couple that truly loves in this way is not withdrawing from the world but learning how to carry the world in concentrated form.